1. Field of the Invention
The present invention generally relates to solid state imagers and, more particularly, to a shape of a micro-condenser lens provided on a sensitive unit of a solid state imager.
2. Description of the Prior Art
In a solid state imager such as a charge-coupled device (CCD) solid state imager, considering a relation among an electrical charge, a noise and an illumination on an image surface on the CCD, it is known that influences of a noise (shot noise) caused by unstable electric charges and a dark field noise are increased on the low illumination side.
In order to reduce the above-mentioned shot noise, it is sufficient to increase an aperture efficiency of the sensitive unit. However, there is a limit on the increase of the above aperture efficiency because the solid state imager has been more and more miniaturized recently. To solve this problem, one such structure for the solid state imager is proposed at present, in which a micro-condenser lens is formed on the sensitive unit. According to this previously-proposed structure in which the micro-condenser lens is formed on the sensitive unit, a utilization factor of light can be increased, a sensitivity on the sensitive unit can be increased and the above shot noise can be effectively reduced. Japanese Laid-Open Patent Publications Nos. 60-53073 and 1-10666 describe methods of forming a micro-condenser lens.
The conventional micro-condenser lens is shaped as in two types of a so-called stripe type in which a single consecutive semi-cylindrical lens B is formed on a group of sensitive units (pixels) 11 with respect to one column or one row as shown in FIG. 1 and of a so-called discrete type in which a lens C is formed on every sensitive unit (pixel) 11 in a separate fashion as shown in FIG. 2. Comparing two kinds of condenser lenses B and C from a standpoint of a utilization efficiency of a light incident on the same point, a light incident on a point Px is converged into an opening 12 of the sensitive unit 11 both in the condenser lenses B and C but a light incident on a point Py cannot be converged into the opening 12 of the sensitive unit 11 according to the condenser lens B of the stripe type. More specifically, in the condenser lens B of the stripe type, the portions thereof other than the sensitive unit opening 12 in the axial direction become invalid portions M for an incident light. In the case of the condenser lens C of the discrete type, the light incident on the point Py can be converged into the sensitive unit opening 12 and therefore, the latter condenser lens C of the discrete type has the advantage such that the utilization factor of light and the sensitivity of the sensitive unit can be increased.
Most of the circumferential edge of the conventional discrete type condenser lens C is shaped as a rectangle and the circumferential edge of the sensitive unit opening 12 is round-cornered in actual practice at corner portions 13 because the opening 12 is formed by the etching-process or the like. Accordingly, as shown in FIG. 2, a horizontal length Ln between a corner portion 14 of the lens C and the corresponding corner portion 13 of the opening 12 is increased so that particularly a light incident on a point of the lens C near its corner portion 14, e.g., a point Pc cannot be converged into the sensitive unit opening 12. Further, if cut surfaces of mountain-like configuration are formed on the corner portions 14 of the lens C in the process of forming the lens C as shown in FIG. 3, then a light incident near a boundary n of the cut surface, i.e., a light incident on each of points point Pd at both sides of boundary n is not converged to the sensitive unit opening 12 but converged to the direction of the adjacent cut surfaces as shown by arrows A and B in FIG. 3. There is then the disadvantage that the utilization efficiency of light is reduced more.